Fuel-injected internal combustion engines are well known, especially for automotive applications. Torque output of such an engine is typically controlled by moderating airflow into the engine via a throttle device. The throttle, usually a butterfly valve disposed at the entrance to the engine intake manifold, may be directly actuated by a driver's foot pedal or may be electronically governed through a digital or analog controller. Under typical driving conditions, the engine is substantially throttled. Because the engine is a positive displacement pump, a vacuum is created in the intake manifold downstream of the throttle valve.
Recently, some engines are known to be provided with means for varying the lift of one or more engine cylinder intake valves to improve fuel economy (also known as variable valve actuation, VVA, and referred to herein as variable valve lift, VVL). Typically, the lift of a plurality of valves in a multiple-cylinder engine is reduced or modulated during operating periods of low engine load to reduce fuel consumption, the amount of lift being directed by an engine control module (ECM) responsive to various performance inputs, operator pedal position, and programmed algorithms.
In some such engines, it is known to control engine torque by directly utilizing the variable valve lift means to controllably throttle the flow of air into each of the individual cylinders, thereby obviating the need for any conventional throttle valve at the inlet to the intake manifold.
A first unfavorable consequence of eliminating a manifold throttle valve is that the air pressure within the manifold is substantially the same as atmospheric pressure outside the engine; i.e., there is no useful level of manifold vacuum. However, a variety of standard engine and other automotive subsystems have evolved over many years which utilize vacuum as the source of actuation. The engine intake manifold has previously been a “free” source of vacuum for operating such devices and functions, which may include brake boosting, evaporative canister purging, exhaust gas recirculation, and HVAC systems among others. Providing an auxiliary vacuum pump for auxiliary automotive devices adds cost to a vehicle, consumes valuable onboard space, and parasitically decreases fuel economy. Engine functions, such as improving fuel preparation for cold starting, inducing exhaust gas recirculation into the intake manifold, and reducing cylinder-to-cylinder air volume differences at light engine loads, require manifold vacuum and cannot be accomplished by addition of an auxiliary vacuum pump.
A second unfavorable consequence of eliminating a manifold throttle valve is that fuel economy typically is sub-optimal when there is no manifold vacuum.
It is a principal object of the present invention to provide a substantially non-parasitic system for creating and managing vacuum for operating vacuum-assisted devices and functions in a vehicle powered by a VVL-equipped engine wherein primary throttling has heretofore been provided exclusively by variable valve lifting.
It is a further object of the invention to provide such a system whereby fuel economy is improved.
It is a still further object of the invention to provide a failsafe means for operating a VVL-equipped and throttled engine in the event that the VVL control fails and the valves assume a full-lift mode.